A terminal apparatus for a superconducting cable system is an apparatus that connects a superconducting cable and a normal conducting cable to each other, i.e., a connecting apparatus for connecting a superconducting cable through which power is transmitted at a cryogenic temperature to an overhead transmission cable or power appliance such as a circuit breaker in an ambient temperature state.
Such a terminal apparatus for a superconducting cable system has been disclosed in Korean Patent No. 10-0508710, Korean Patent No. 10-0590200, Korean Patent No. 10-0642538, and Korean Patent Application Publication No. 10-2007-0003879.
FIG. 1 illustrates a schematic view of a related art terminal apparatus for a superconducting cable system, disclosed in the aforementioned patents and the like. As illustrated in FIG. 1, the terminal apparatus 10 for the superconductor cable system includes a refrigerant tub 30 filled in the interior thereof with a refrigerant 31 such as liquid nitrogen while being connected to an end portion of a superconducting cable 20, a vacuum heat insulating container 40 that surrounds the exterior of the refrigerant tub 30, a nonconductor 50 made of a porcelain tube (insulator) or the like, disposed at an ambient temperature side of the vacuum heat insulating container 40, and a current lead 60 having one end connected to the end portion of the superconducting cable 20 through a conductor connecting portion 21 and the other end extended to the interior of the nonconductor 50 by passing through the refrigerant tub 30 and the vacuum heat insulating container 40 so as to be connected to an overhead transmission cable or power appliance.
In the related art terminal apparatus 10 for the superconducting cable system, a general metal such as copper or aluminum is used as the current lead 60. However, since electrical resistance is constant regardless of current in the general metal such as copper or aluminum, transient fault current flows into the superconducting cable 20 or external power appliance through the current lead 60 as it is, when a power transmission fault occurs. Therefore, the superconducting cable 20 or external power appliance may be damaged.
When the fault occurs, the temperature of an extremely lower temperature refrigerant (liquid nitrogen or the like) is increased by the heat generation of the current lead 60 due to the fault current. In this case, the heat generation may be limited to some degree by increasing the sectional area of the current lead 60. However, an amount of heat (ambient temperature) is further conducted as much as the increased sectional area from an ambient temperature side (upper portion of the nonconductor 50 in FIG. 1) to a cryogenic temperature side (refrigerant side), and therefore, the temperature of the refrigerant may be increased.
Meanwhile, as power systems are complicated due to the increase of power demand, fault current caused by faults of the power systems is gradually increased, and accordingly, a large-scale circuit breaker is required. If a superconducting fault current limiter (SFCL) is applied corresponding to the circuit breaker, the SFCL can function to protect the power systems, e.g., to decrease the capacity of the circuit breaker by reducing the amplitude of the fault current, and the like.
However, since the related art SFCL is established separately from the terminal apparatus for the superconducting cable system, it is quenched due to the occurrence of a power transmission fault in a power system, and excessive heat is generated when fault current is limited. Accordingly, the temperature of an internal refrigerant is excessively increased, and the refrigerant may be vaporized. For example, if a power transmission fault occurs in a power system when an SFCL is used, an operation is performed through a sequence of the occurrence of a power transmission fault, the limitation of fault current in the SFCL, the operation of a circuit breaker (limitation of power) and the reoperation of the circuit breaker (reconnection of power). Therefore, the SFCL is necessarily restored to a normal state before the reoperation of the circuit breaker. To this end, it is necessarily required to use a large-scale cooling apparatus (refrigerant apparatus) capable of absorbing the amount of heat generated by limiting the fault current. Further, it is necessary to protect against a pressure fault caused by the volume expansion of a refrigerant such liquid nitrogen. Furthermore, since the related art SFCL is provided only to protect the power system, it is necessary to add a protecting function in the design of a superconducting cable system.